198 STUDIES IN EVOLUTION 



In their present condition the specimens contain very little 

 calcite matter, and nearly the entire calcareous and chitinous 

 portions of the animal are represented by a thin film of iron 

 pyrite. To this kind of fossilization is doubtless due the 

 preservation of delicate organs and structures which other- 

 wise would have been destroyed. For, as is well known, 

 pyrite may replace such organic tissues as chitine or even 

 soft dermal structures, the change occurring by the slow 

 decomposition of these tissues in the presence of iron sul- 

 phate in solution, or from the action of hydrogen sulphide as 

 a result of decomposition in a chalybeate water. 



From the mode of occurrence of the specimens it is evident 

 that some physical change of a rather sudden nature must be 

 inferred to explain the facts. This is shown from the follow- 

 ing considerations : (1) Their restricted vertical distribution ; 

 (2) nearly all specimens are complete and preserve their 

 appendages; (3) they are of all ages, from larval forms up 

 to full-grown individuals; (4) the rock has a characteristic 

 structure and composition; and (5) the adjacent strata con- 

 tain a rather sparse fauna in which the trilobites are generally 

 fragmentary, or usually without appendages. 



It does not require a violent catastrophe to account for 

 these peculiarities, and, as in the case of the recent destruc- 

 tion of the tile-fish off the eastern coast of the United States, 

 it is possible that a temporary change in the direction of an 

 ocean current, with the consequent variation of temperature, 

 would be amply sufficient. Just what occurred in the present 

 instance has not been determined. Throughout the trilobite- 

 bearing rocks generally, young and larval forms are extremely 

 rare, while, of full-grown examples, fragments are the rule 

 and entire specimens the exception. Therefore it is believed 

 that the remains commonly found represent sheddings or 

 moults, and not in each case the death of a separate indi- 

 vidual. In the present material, however, the almost in- 

 variable perfection of the specimens precludes this vicAV. 

 Moreover, the appendages are apparently in the position held 

 in life, and not such as obtain in the cast-off shells of recent 

 Crustacea. 



